Typical complementary metal-oxide-semiconductor (CMOS) image sensors sense light by converting impinging photons into electrons that are integrated (collected) in sensor pixels. Once the integration cycle is complete, collected charge is converted into a voltage signal, which is supplied to output terminals of an image sensor. This charge to voltage conversion is performed within each sensor pixel. The pixel output voltage (i.e., an analog voltage signal) is transferred to the output terminals using various pixel addressing and scanning schemes. The analog voltage signal can also be converted on-chip to a digital equivalent before reaching the chip output.
The sensor pixels include buffer amplifiers (i.e., source followers) that drive sensing lines connected to the pixels through address transistors. After the charge to voltage conversion and after the resulting voltage signal has been read out from the pixels, the pixels are reset in preparation for a successive charge accumulation cycle. In pixels that include floating diffusions (FD) serving as charge detection nodes, the reset operation is performed by turning on a reset transistor that connects the floating diffusion node to a voltage reference.
Removing charge from the floating diffusion node using the reset transistor, however, generates kTC-reset noise as is well known in the art. The kTC noise must be removed using correlated double sampling (CDS) signal processing technique in order to achieve desired low noise performance. Typical CMOS image sensors that utilize CDS typically require four transistors (4T) per pixel. An example of the 4T pixel circuit with a pinned photo-diode can be found in Guidash (U.S. Pat. No. 5,991,184), incorporated herein as a reference.
Color sensing in typical single-chip CMOS and charge-coupled device (CCD) image sensors is accomplished by placing light-absorbing/color-transmitting filters over the sensor pixels in a predetermined pattern. The different pixels in a given pixel sub-group or sub-array is therefore sensitive to a certain wavelength band of the electromagnetic spectrum. Signals gathered using the different pixel subgroups that have different color sensitivity are then used to construct super-pixel signals using various interpolating and signal processing schemes in an attempt to recover the resolution that has been lost as a result of using color filters.
Examples of conventional color filter array patterns are found in Bayer (U.S. Pat. No. 3,971,065) and Kasano (“A 2.0 um Pixel Pitch MOS Image Sensor with an Amorphous Si Film Color Filter,” Digest of Technical Papers ISCC, vol. 48, February 2005, pp. 348349), incorporated herein as references. The color filtering schemes as described in Bayer and Kasano may undesirably sacrifice resolution and sensitivity as a result of light absorption when color filters are used.
In an effort to counteract this reduction in resolution and sensitivity, stacked image sensor pixels that have three photo-diodes placed above each other have been developed by Foveon (see, Merrill U.S. Pat. No. 6,894,265, incorporated herein as a reference). No color filter is placed over the image pixels of Merrill. The three photo-diodes are formed in a silicon substrate at different depths. Impinging photons entering the image sensor may generate carriers in the silicon substrate. The generated carriers are collected at the different depths by the corresponding photo-diodes. A voltage signal is then obtained by connecting the three buried photo-diodes (i.e., the photo-diodes are “buried” in the substrate) to circuitry formed over the surface of the silicon substrate. The voltage signal is sensed, processed, and reset using conventional image pixel readout/reset operations.
Forming image sensors using this approach provides improved resolution and sensitivity because no color filters are used (i.e., no photons are absorbed in the color filters). It may, however, be difficult and costly to form three photo-diodes that are buried deeply in the silicon substrate. It may also be challenging to sense charge collected by all three buried photo-diodes without adding noise.
It would therefore be desirable to be able to provide image sensors that exhibit an improved resolution, sensitivity, and noise.